After 4 years and 250+cycles, my original battery was on its last legs. I took it to the Apple store, where I was told that they would NOT replace the battery (for $130) because there was evidence that there had been a liquid spill into the unibody -- but they WOULD be happy to charge me $700 to clean everything out, bring the parts back to 2009 spec and replace the battery...
Smooth as silk. Took less than 30 minutes, and cost $50 less than having Apple do it -- if they WOULD do it (which they wouldn't).
Follow the instructions, and all will be well.